Since flame resistant fiber is excellent in heat resistance and flame retardancy, it has been widely used, for example, for spatter sheets for protecting human body from high-temperature iron powder and weld sparks which are scattered in welding operation, and fire-resistant heat insulators for aircrafts, and its demand in these fields are increasing.
Further, the flame resistant fiber is important also as intermediate raw materials for obtaining carbon fiber. Carbon fiber has been, due to such various mechanical, chemical properties and lightweight properties, widely employed for various applications, for example, materials for aviation and space such as aircrafts and rockets, and sporting goods such as tennis rackets, golf shafts and fishing rods, and to be used also in fields for transport machines such as ships and cars. In addition, in recent years, the carbon fiber has been, due to its high electrical conductivity and heat releasing property, strongly required for application to electronic equipment parts such as housings for cellular phones and personal computer, and to electrodes of fuel cells.
Such a carbon fiber is generally obtained by a carbonization treatment of a flame resistant fiber by heating at a high temperature in an inert gas such as nitrogen. With regard to conventional flame resistant fiber, for example, a polyacrylonitrile (hereunder, abbreviated as PAN) based flame resistant fiber is obtained by making a PAN-based precursor fiber flame resistant (cyclization reaction and oxidation reaction of PAN) at a high temperature of 200 to 300° C. in the air.
However, this reaction to make flame resistant is an exothermic reaction and a reaction in a fibrous form, namely, in a state of solid-phase. Therefore, a long-time treatment is required for its temperature control, and the fiber thickness of the PAN-based precursor fiber needs to be limited to a fine size below a specific value to finish making flame resistant within a desired time. Thus, the presently known process of making flame resistant is unlikely to be regarded as a sufficiently efficient process.
As a method for solving the above-mentioned technical problems, a method of making solution by a solvent has been studied.
For example, a technique is disclosed in which an acrylonitrile- (hereunder, abbreviated as AN) based polymer powder is heated in an inert atmosphere until its density becomes 1.20 g/cm3 or more, and thereafter dissolved in a solvent and made into a fiber, and the fibrous material is heat-treated (for example, refer to JP-S63-14093B).
However, there was a problem that, because an acrylonitrile polymer powder not well made into flame resistant is used, since viscosity change with time of the solution is big, a yarn breakage is likely to occur. Since a strongly acidic solvent such as sulfuric acid or nitric acid for easily decomposing ordinary organic polymers was used, an apparatus made of special materials having corrosion resistance needs to be used, etc., it was not practical in cost, too.
Further, a method is proposed such that a heat-treated acrylonitrile polymer powder and a not heat-treated acrylonitrile polymer powder are mixed and similarly dissolved in an acidic solvent (for example, refer to JP-S62-57723B), but the problem was still not solved as to providing corrosion resistance to the apparatus as described above and as to the instability of solution.
In addition, it is disclosed that, by heat-treating a PAN solution in dimethylformamide, the PAN is converted to a polymer having a cyclized structure (for example, refer to “Polymer Science (USSR),” (Polym. Sci. USSR), 1968, Vol. 10, page 1537), however, since it is such a dilute solution that the polymer concentration is 0.5% and so low in viscosity as to be substantially difficult in forming or molding into a fiber or the like, and a rise in concentration thereof caused the polymer to be precipitated to make incapable of being used as a solution.
On the other hand, a solution in which PAN is modified with a primary amine is disclosed (for example, refer to “Journal of Polymer Science, Part A: Polymer Chemistry,” (J. Polym. Sci. Part A: Polym. Chem.), 1990, Vol. 28, page 1623), but the solution is such as that imparted with hydrophilic property to the PAN itself, which is not made into flame resistant, and totally differs in technical ideas from a solution containing flame resistant polymer.
Further, a technique in which the yield can be improved together with physical properties by converting a flame resistant fiber to a carbon fiber in a specific carbonizing condition is disclosed (for example, refer to Official Gazette of JP Patent No. 2636509), but a compatibility therebetween with an easier method has been demanded.
With regard to a flame resistant polymer soluble in an organic solvent, we made a proposal (Pamphlet of WO 05/080448 A). However, at producing the flame resistant fiber by employing the technique, a more stabilized fiber forming ability, an improvement of physical properties of the obtained flame resistant or carbon fiber, decrease of variation of physical properties between single fibers in those fiber assemblies, or the like have been desired.
Furthermore, the flame resistant fiber obtained by the method described in Pamphlet of WO 05/080448 A, compared to the flame resistant fiber produced by the conventional step of making flame resistant in the air, could not be said that its mechanical strength is necessarily sufficient. The reason is, although it is known that excellent mechanical characteristics are exhibited by arranging orientation of constituent polymer in fiber axis direction, in the case of the method described in Pamphlet of WO 05/080448 A, that the flame resistant polymer as extruded from a spinneret is in a state of no orientation, and on the other hand, it was extremely difficult to make the molecule highly oriented by a drawing in the process since the flame resistant polymer molecule has a rigid structure.
For example, it is the major trend that carbon fibers used as a primary structural material of aircrafts are those in which PAN is used as the starting material. That is because the fiber is excellent in tensile modulus and strength in fiber axis direction, further, since it is also high in compressive strength, the material exhibits a high rigidity and, simultaneously, a defect such as a yield is unlikely to occur when molded into a composite. On the other hand, compared to the PAN-based carbon fiber, pitch-based carbon fiber has characteristics that its tensile modulus is high and its thermal conductivity or electric conductivity is also high, and used as material for panels of artificial satellite, cement reinforcement, parts of printer or copying machine, or the like.
Here, the difference of characteristics of both fibers is based on a difference of aggregate structures of graphene as described above, but the most basic unit of the aggregate structure of grapheme is a crystal. A well known relation between a crystal structure and characteristics is a relation between modulus or compressive strength and a crystal size. In general, in carbon fibers, as crystal size becomes large, modulus increases. This is because the modulus is related to structural regularity and it is understood that, as the crystal size increases and the structural regularity becomes higher, molecular movement by an external stress becomes smaller and the modulus becomes higher. When a fiber is compressed in the fiber axis direction, the maximum shear stress is loaded in direction of 45° to the fiber axis. This shear stress functions to crystal surfaces to slide, but since the force present between the graphenes is the van der Waals force which is very weak, when the crystal is large, a breakage is easy and likely to occur. That is, when crystal is large, a trade-off characteristic that modulus is high but compressive strength is low appears. A solution of the trade-off is a desire for long years in carbon fiber, and a compatibility of the modulus and the compressive strength is a strongly desired problem to be solved.
Furthermore, when electric conductivity or thermal conductivity is to be improved, it is advantageous that the crystal size is large. This is because the crystal transmits electrons in carbon fiber, and it is advantageous to make the crystal, which is the path of the electrons, as large as possible to maintain its continuity. On the other hand, when the crystal size is made large to increase electric conductivity or thermal conductivity, a defect is brought about that the material strength decreases and becomes brittle. That is because, in general, when crystal size is made large, to the extent that molecules are taken into the crystal, the space where the molecules were present becomes a void to generate a structural defect.
As mentioned above, with regard to carbon fiber, there are some problems of trade-off relation which are based on its structure and theoretically difficult to be solved. In order to solve these problems, various efforts have been made for long years. For example, with regard to compatibility between the modulus and the compressive strength of the PAN-based carbon fiber, the following technique is disclosed.
For example, when an acryl-based fiber is subjected to a carbonization treatment, a technique for obtaining a carbon fiber having a high tensile modulus and a high compressive strength by raising the temperature to 2200° C. or more and positively drawing the fiber to prevent its orientation relaxation, is disclosed. However, due to the high heat treatment temperature, the crystal size of the carbon fiber becomes large and the compressive strength is not increased as expected (refer to JP-S63-211326 A).
Further, a technique for decreasing crystal size by an electron beam radiation is disclosed (refer to JP-H4-19219 A). This is a technique for lowering molecular movement by electron beam cross-linking and preventing crystal growth. According to this technique, although the crystal size of the carbon fiber in surface layer of the carbon fiber decreases, since the maximum temperature of the carbonization treatment becomes 2000° C. or more, the crystal size of the carbon fiber as a whole does not decrease, and the improvement of the compressive strength was insufficient.
As the above-mentioned, various methods have been studied to improve carbon fiber characteristics by controlling a draw ratio or the like during heat treatments such as at fiber formation or carbonization treatment. However, in spite of these efforts, a PAN-based carbon fiber of which tensile modulus and compressive strength are compatible in a high level has not been obtained.
In the case of pitch-based carbon fiber which is another representative carbon fiber, the following technique is being studied to improve its weak point which is compressive strength.
For example, a technique of improving compressive strength of carbon fiber by spinning a pitch containing 5 to 40% optically anisotropic phase at an extremely high spinning viscosity (several thousands poises), and this is carbonized (refer to JP-H2-14023 A). A method of improving compressive strength by injecting boron ion under vacuum to a pitch-based carbon fiber is disclosed (refer to JP-H3-816 A).
However, in these techniques, compared to conventional production methods of carbon fiber, an extremely specific process condition or an impractical process means is necessary, and it was difficult to employ them as an industrial pitch-based production method of carbon fiber.
The above-mentioned are techniques relating to improvement of compressive strength, but there are many other problems of long years to be solved and they are too many to be mentioned. Various efforts have been made for both of the PAN-based and the pitch-based carbon fibers and the physical properties have been tried to be improved by process conditions or by improvement of precursor/carbon fiber, but any of them has brought about a substantial improvement.
By the way, we propose a production method completely different from the above-mentioned conventional carbon fiber produced by using the flame resistant fiber obtainable by the air-oxidation, i.e., a production method of carbon fiber of which starting material is an amine-modified flame resistant polymer (refer to Pamphlet of WO 05/080448 A). In the production method of carbon fiber described in the patent reference, there are merits that the production process of the flame resistant polymer which is the starting material is, compared to the production process of the conventional flame resistant fiber, not only low in plant cost, but also excellent in safety and in working environment. However, the flame resistant polymer molecule to be the starting material has a rigid structure, and it is extremely difficult to make this molecule highly oriented, and as a result, it was also difficult to obtain a carbon fiber excellent in mechanical properties.
Thus, it would be advantageous to provide a flame resistant fiber obtainable by fiber forming a flame resistant polymer, to improve fiber forming ability to obtain a flame resistant fiber assembly of a higher performance.